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Friday, 2 February 2018

EARTHQUAKES RESISTANT BUILDINGS

An
earthquake is a sudden, rapid shaking of the earth surface caused by the
breaking and shifting of rocks beneath. During earthquake, ground motion occurs
in a random fashion in all directions radiating from a point within earth
crust, called epicentre. It causes vibrations of structures and induce inertia forces
on them. As a result structure may collapse resulting into loss of property and
lives. Earthquakes do not kill people, vulnerable buildings do so. Hence there
is need of designing earthquake resistant buildings, especially in the
earthquake prone areas.

TYPES OF EARTHQUAKES

Depending
upon the possible causes, the earthquakes may be classified as:

1.
Natural earthquake

2. Earthquakes due to
induced activities.

Natural Earthquakes

Natural
earthquakes may be due to

(i) active faults (ii) movement of tectonic plates or

(iii) due to volcanic eruptions.

In
earth’s crust there are some faults which are not yet settled. The displacement
of rocks along

faults
cause earthquake.

Tectonic
means large scale process affecting the structure of the earthcrust. This
process causes

gradual
movement of material within the crust of earth. Sometimes it shakes the earth
crust.

Volcano
is a mountain or hill having a crater through which lava, rock fragments, hot
vapour and

gas
are or have been erupted from the earths crust. Occasionally the volcanoes
become active and create earthquake near the mountain crater.

Earthquakes due to Induced Activities

These
are caused by vibrations induced by atomic explosions and collapse of ground
due to

faulty
mining.

TERMINOLOGY

1.
Focus: The point on the fault where slip starts is the focus. It is also known
as hypocentre

[Ref.
Fig. 20.1].

2. Epicenter:
The point vertically above the focus on the surface of the earth is the
epicentre.

3.
Focal Depth: The depth of focus from the epicentre is called the focal depth.

4.
Epicentral Distance: Distance from epicentre to any point of interest on the
surface of earth is

called epicentral
distance.

MAGNITUDE AND INTENSITY

Magnitude is a quantitative
measure of the actual size of the earthquake. Professor Charles Richter

proposed
the scale of magnitude that goes from 0 to 9. It is a geometric scale. Now this
scale is known as Richter scale. It is obtained from the seismograph. It
depends on waveform amplitude on epicentral distance. It is denoted by letter M
followed by the number. An increase in magnitude by 1 implies 10 times higher
waveform amplitude and about 31 times higher energy released. Thus energy
released in M6 and M5 earthquake have the ratio 31, and M8 to M5 have the ratio
31 × 31 × 31. There are other magnitude scales, like the Body Wave Magnitude,
Surface Wave Magnitude and Wave Energy Magnitude. Intensity is a qualitative
measure of the actual shaking at a location during an earthquake. Hence for the
same earthquake, it has different values at different places, highest value
being at epicentre. This is a linear scale. It is assigned as Roman Capital
Numbers from I to XII. Intensity depends upon

1.
Amount of source energy released

2.
Distance between the source and the place of interest

3.
Geographical features of the media of travel and importantly on the type of
structure.

After
observing Indian earthquakes for several years Bureau of Indian Standard has
divided the country into five zones depending upon the severity of earthquake.
IS 1893-1984 shows the various zones. The following IS codes will be of great
importance for the design engineers: